1. Field of the Invention
The present invention relates to liquid crystalline compounds effective as components of liquid crystal compositions, particularly, dioxane derivatives preferably used as liquid crystal compositions for TFT, liquid crystal compositions containing them, and liquid crystal display devices constituted by using these derivatives.
2. Description of the Prior Art
Liquid crystal display devices are obtained by using liquid crystal materials having properties of optical anisotropy and dielectric anisotropy. The display modes are twist nematic (TN) mode, dinamic scattering (DS) mode, guest-host (G.H) mode, deformation of aligned phases (DAP) mode, super twist nematic (STN) mode and the like. The properties of liquid crystal materials are different in each mode. Lately, in particular, liquid crystal devices having high display quality are demanded. In response to the demand, a display device of an active matrix mode represented by a thin film transistor (TFT) mode have been used increasingly.
The liquid crystalline compounds used for any display devices should be stable to moisture, air, heat, light and the like. These compounds also should have a liquid crystal phase over a wide range of temperatures around room temperature, and have low viscosity, good compatibility to the other liquid crystalline compounds and liquid crystal compositions, a high dielectric anisotropy value (.DELTA..epsilon.), and proper optical anisotropy (.DELTA.n). These compounds should be stable chemically and electrically. Particularly, the display device of the active matrix mode represented by the TFT mode requires a high voltage holding ratio. However, there are no materials satisfying these conditions, so that liquid crystal compositions, which are obtained by mixing several kinds of liquid crystalline compounds or liquid crystalline compounds, are used under existing circumstances.
Lately, low voltage driving is demanded to the liquid crystal device of the TFT mode. In response to the demand, liquid crystalline compounds and liquid crystal compositions, which have .DELTA..epsilon. higher than that of the liquid crystal materials used for conventional liquid crystal display devices of the TFT mode, are required. As a result, liquid crystal materials having a high voltage holding ratio and high .DELTA..epsilon. are actively developed. Hitherto, it has been generally known that liquid crystal materials of fluorine types show a high voltage holding ratio, and Japanese Patent Publication No. 01-04496 discloses the following compound (10). ##STR2##
Since the above compound (10) has a voltage holding ratio higher than that of a liquid crystalline compound having a cyano group, it is mainly used as a component of the liquid crystal composition for TFT. However, extrapolated .DELTA..epsilon. of compound (10) (extrapolated .DELTA..epsilon. means that it is calculated from the value of .DELTA..epsilon. of the composition, dissolving the compounds in a mother liquid crystal having a nematic phase, the value of .DELTA..epsilon. of the mother liquid crystal and the mixture ratio, and the .DELTA..epsilon. described hereinafter shows the extrapolated .DELTA..epsilon.) is small as 8.5. The compound can not be used as liquid crystal materials for low voltage driving, which is represented by 2.5 V driving and demanded in practice.
As a compound having .DELTA..epsilon. higher than that of compound (10), Japanese Patent Laid-Open Publication No. 02-233626 discloses the trifluorophenyl compound (11) represented by the following formula: ##STR3##
Although .DELTA..epsilon. of compound (11) is 11.0 and higher than that of compound (10), the value is still too small to satisfy the demand of lower voltage like the above described value.
Japanese Patent Laid-Open Publication No. 04-506361 discloses trifluoromethylphenyl derivative (12) and trifluoromethoxyphenyl derivative (13). ##STR4##
However, .DELTA..epsilon. of these compounds can not satisfy the demand of lower voltage in the market like compounds (10) and (11). (For example, .DELTA..epsilon. of compound (13) is about 5 (IDY (Televigakugihou) 95).
As a compound having higher .DELTA..epsilon., Japanese Patent Laid-Open Publication No. 02-233626 discloses a dioxane derivative represented by the following formula (14): ##STR5##
Although .DELTA..epsilon. of the compound is high 15.7, it is impossible to lower the driving voltage into a necessary level. Moreover, the voltage holding ratio is smaller than that of liquid crystalline compounds of fluoric types not having a dioxane ring, so that the compound can not be contained in the materials for a liquid crystal display devices demanding higher voltage holding ratio As an example, the voltage holding ratio of compound (11) is 98% at a temperature of 25.degree. C. and 96% at 100.degree. C., while that of compound (14) is 98% at 25.degree. C. and 92% at 100.degree. C.
Accordingly, liquid crystalline compounds and liquid crystalline compounds having both higher .DELTA..epsilon. and higher voltage ratio have been desired eagerly.
The inventors of the present invention has studied earnestly to find a liquid crystalline compound most suitable for liquid crystal materials for the TFT low voltage driving represented by 2.5V driving, and they have found that the liquid crystalline compound, in which a Sp3 carbon is bound to the 2-position of a 1,3-dioxane-2,5-diyl group, and the molecular end has an electron-attracting group of a fluorine or chlorine type, has specifically high .DELTA..epsilon. and high voltage holding ratio.